Primer on Fiber Optic Data Communications for the Premises Environment

by

 

Dr. Kenneth S. Schneider

Table of Contents

Acknowledgements

1. Introduction
    1.1 The Fundamental Problem of Communication
    1.2 The Transmission Medium - Attenuation Constraints
    1.3 The Transmission Medium - Interference Constraints
    1.4 The Transmission Medium - Bandwidth Constraints
    1.5 The Transmission Medium - Cost Constraints
    1.6 Attractiveness of Fiber Optic Cable As A Premises Transmission Medium
    1.7 Program

2. The Fiber Optic Data Communications Link For the Premises Environment
    2.1 The Fiber Optic Data Communications Link, End-to-End
    2.2 Fiber Optic Cable
    2.3 Transmitter
    2.4 Receiver
    2.5 Connectors
    2.6 Splicing
    2.7 Analyzing Performance of a Link

3. Exploiting The Bandwidth Of Fiber Optic Cable-Employment by Multiple Users
    3.1 Sharing the Transmission Medium
    3.2 Time Division Multiplexing (TDM) With Fiber Optic Cable
    3.3 Wavelength Division Multiplexing (WDM) With Fiber Optic Cable
    3.4 Comparing Multiplexing Techniques for the Premises Environment

4. Exploiting The Delay Properties Of Fiber Optic Cable For LAN Extension
    4.1 Brief History of Local Area Networks
    4.2 Transmission Media Used To Implement An Ethernet LAN
    4.3 Examining the Distance Constraint
    4.4 Examples of LAN Extenders Shown In Typical Applications

5. Exploiting The Advantages Of Fiber Optic Cable In the Industrial Environment
    5.1 Data Communications In The Industrial Environment
    5.2 The Problem of Interference
    5.3 Fiber Optic Data Communications Products That can Help

6. Serial Data Communications Over Fiber Optic Cable

7. Standards

8. Glossary

Bibliography

ACKNOWLEDGEMENTS

ACKNOWLEDGEMENTS

The idea for writing a monograph on the subject of fiber optic data communications was proposed to me many times by my assistant, Gail Nelson.

The material in this work was derived from my constant perusal of many diverse sources spread over my years in engineering. I apologize for not providing a precise acknowledgment of every source. However, it would have led to a clutter of footnotes. I know that this often makes for tedious reading and did not want to burden the reader. Nonetheless, I would not feel comfortable unless specific credit is given to those publications listed as 'References.' If, on occasion, I paraphrased any of these works too closely it should be taken in the most complimentary manner.

Pat O'Hara assisted me in taking a typed manuscript and putting it in final form complete with graphics, photographs and other illustrations. Pat carries out this task for all of my publications. She never complains when I come to her with last minute changes. Her cooperation is really appreciated. I can truthfully say this work would not have been completed without her assistance. Note to Pat, we'll soon begin another effort.

Thanks to Doug Honikel for having incorporated this onto our website.

Tony Horber and Bob Ravenstein (Bomara, Inc.) checked the work for technical accuracy. This was a particularly stressful task especially when it led to protracted discussions on certain points. I am indebted to them for their efforts.

Professor Nicholas DeClaris first introduced me to communications engineering while I was an undergraduate at Cornell University. Professor DeClaris, now of the University of Maryland, inspired me with his love for teaching and research. Dr. Irvin Stiglitz later sharpened my communications engineering and technical writing skills while he was my Group Leader at M.I.T. Lincoln Laboratory. Needless to say, it is a lot easier to reach Irv's high standards these days with word processing.

Thanks to Lightwave Magazine and MRV Communications for use of the illustration for the cover.

Finally, I would like to thank my wife, Diane, my children Andrew, Jessica and Rachel, my mother and father, Lillian and Irving Schneider and my, close, life long, friends Seth Stowell, Jamil Sopher and Joel Goldman. In different ways each gave me encouragement over the years. Without this support I would have never have reached this point.

*ST is a registered trademark of AT & T

CHAPTER 1

INTRODUCTION

1.1 The Fundamental Problem of Communications

The subject of interest in this book is premises data communications using fiber optic cable as the transmission medium. This is at once a very specific yet very extensive topic. It is an important topic both within the context of data communications today and into the future. All, or almost all, aspects of this subject will be explored. However, it seems rather forbidding just to jump into this topic.

Rather, it is more appropriate to take a step back to the very beginning and talk about the nature of communications first. This will allow some needed terminology to be introduced. It will also lead us in a natural way to the subject of fiber optic cable as a transmission medium and to why it is attractive for premises data links.

Of course, the reader, well versed in data communications, may choose to skip past this introduction and suffer no real penalty.

The subject of communications really begins with the situation shown in Figure 1-1. Here is an entity called the Source and one called the User- located remotely from the Source. The Source generates Information and the User desires to learn what this Information is.

 

Figure 1-1: Source, User pair with information

Examples of this situation are everywhere prevalent. However, our attention will only be focused on the case illustrated in Figure 1-2 where the Information is a sequence of binary digits, 0's and 1's, bits. Information in this case is termed data. Information of this type is generally associated with computers, computing type devices and peripherals-equipment shown in Figure 1-3. Limiting Information to data presents no real limitation. Voice, images, indeed most other types of Information can be processed to look like data by carrying sampling and Analog-to-Digital conversion.

 

Figure 1-2: Representations of information

 

 

Figure 1-3: Examples of sources and users generating/desiring "data"

It is absolutely impossible in the real world for the User to obtain the Information without the chance of error. These may be caused by a variety of deleterious effects that shall be discussed in the sequel.

This means that the User wanting to learn the Information- the binary sequence- must be content in learning it to within a given fidelity. The fidelity measure usually employed is the Bit Error Rate (BER). This is just the probability that a specific generated binary digit at the Source, a bit, is received in error, opposite to what it is, at the User.

There are some real questions as to how appropriate this fidelity measure is in certain applications. Nonetheless, it is so widely employed in practice, at this point, that further discussion is not warranted.

The question then arises as to how to send the binary data stream from Source to User. A Transmission Medium is employed to transport the Information from Source to User. What is a Transmission Medium?

A Transmission Medium is some physical entity. As shown in Figure 1-4 it is located between the Source and the User and it is accessible to both. The Transmission Medium has a set of properties described by physical parameters. The set of properties exists in a quiescent state. However, at least one of these properties can be stressed or disturbed at the Source end. This is accomplished by somehow imparting energy in order to stress the property. This disturbance does not stay still, but affects the parts of the Transmission Medium around it. This disturbance then travels from the Source end to the User end. Consequently, energy imparted in creating the disturbance is thereby transferred from the Source end to the User end. Finally, this disturbance or stressed property, can be sensed at the User end. It can be measured.

Figure 1-4: Source, transmission medium, user

This propagation of a disturbance by the Transmission Medium is illustrated in Figure 1-5.

What are examples of transmission media? As with types of Information there are many.

 

Figure 1-5: Disturbance traveling in transmission medium

The Transmission Medium could be air with the stressed property being the air pressure sound waves. The Transmission Medium could be an electromagnetic field set up in space by the current put on an antenna, a radio or wireless system. The Transmission Medium could be a pair of electrical conductors with the stressed property being the potential difference (the voltage) between the conductors, an electrical transmission line. The Transmission Medium could be a sheet of writing paper with the stressed property being the light-dark pattern on the paper, a letter. The Transmission Medium could be a cylindrical glass tube with the stressed property being the intensity of light in the tube, a fiber optic cable.

The Source can have a disturbance to the Transmission medium generated in sympathy to the Information, that is, generate a disturbance which varies in time exactly as the Information. This encoded disturbance will then propagate to the User. The User can then sense the disturbance and decide the identity of the Information that it represents. The process of the Source generating a disturbance in sympathy with the Information and launching it into the Transmission Medium is referred to as modulation and transmission. The process of the User sensing the received disturbance and deciding what Information it represents is referred to as reception and demodulation. The device that carries out modulation and transmission will be called in this work the Transmitter. The device that carries out reception and demodulation will be called the Receiver.

The entire situation with data communications then devolves to the model illustrated in Figure 1-6. Here the Source is generating bits as Information. The User wants to learn the identity of this Information, these bits. The entities used to get the Information from the Source to User are the Transmitter, the Transmission Medium and the Receiver. The fundamental problem of communications is to choose the terminal equipment, the Transmitter and Receiver and to choose the Transmission Medium so as to satisfy the requirements for a given Source-User pair.

 

Figure 1-6: The model which represents the fundamental problem of communications

The fundamental problem of communications is a design problem. The combination of Transmitter, Transmission Medium and Receiver is termed the communication link. Because of the limitation placed on the Information to be a sequence of bits this combination is generally referred to as a data link. The disturbance launched into the Transmission Medium by the Transmitter is usually referred to as the input data signal. The resulting disturbance at the Receiver is termed the output data signal. In the context of our discussion the fundamental problem of communications is to design a data link appropriate for connecting a given Source-User pair.

There is no fail safe cookbook way to solve this design problem and come up with the best unique solution. While there is science here there is also art. There are always alternative solutions, each with a particular twist. The twist provides some additional attractive feature to the solution. However, the feature is really peripheral to Source-User requirements.

Most exercises in obtaining the design solution usually begin with choosing a Transmission Medium to meet the general requirements of the Source-User pair. That is, the data link design process pivots on choosing the Transmission Medium. Every Transmission Medium has constraints on its operation, on its performance. It is these constraints that really decide which Transmission Medium will be employed for the data link design. It will be worthwhile discussing these constraints.

1.2 The Transmission Medium- Attenuation Constraints

Have a Transmitter launch a disturbance into a Transmission Medium. Provide an input data signal to a Transmission Medium. As it propagates down the Transmission Medium to the Receiver its amplitude will decrease, getting weaker and weaker. The disturbance, the input data signal, is said to suffer attenuation. The situation is exactly as shown in Figure 1-7.

One immediate question that can be raised is why does attenuation occur? There are several reasons. It will be worthwhile pointing out and describing two of them; spatial dispersion and loss due to heat.

Spatial dispersion can best be considered by revisiting Figure 1-7. This illustrates a one-dimensional propagation of the disturbance. However, often this disturbance may propagate in two or even three dimensions. The User/Receiver may be located in a small solid angle relative to the Source/Transmitter. The received disturbance, the output data signal, appears attenuated relative to the transmitted disturbance because in fact, it represents only a small fraction of the overall energy imparted in the disturbance when it was launched. This is exactly the situation with free space propagation of waves through an electromagnetic field transmission medium. For example, this occurs in any sort of radio transmission.

 

Figure 1-7: Input data signal attenuating as it propagates down a transmission medium

As for loss due to heat, this refers to the basic interaction of the disturbance with the material from which the Transmission Medium is comprised. As the disturbance propagates, a portion of the energy is transferred into the Transmission Medium and heats it. For a mechanical analogy to this consider rolling a ball down a cement lane. The ball is the disturbance launched into the lane that represents the Transmission Medium. As the ball rolls along it encounters friction. It loses part of its kinetic energy to heating the cement lane. The ball begins to slow down. The disturbance gets attenuated. This is the situation with using the potential difference between a pair of electrical conductors as the Transmission Medium.

Attenuation increases with the distance through the Transmission Medium. In fact, the amplitude attenuation is measured in dB/km. As propagation continues attenuation increases. Ultimately, the propagating signal is attenuated until it is at some minimal, detectable, level. That is, the signal is attenuated until it can just be sensed by the Receiver- in the presence of whatever interference is expected. The distance at which the signal reaches this minimal level could be quite significant. The Transmission Medium has to be able to deliver at least the minimal detectable level of output signal to the Receiver by the User. If it can not, communications between Source and User really can not take place.

There are some tricks to getting around this. Suppose the disturbance has been attenuated to the minimal detectable level yet it has still not arrived at the Receiver/User. The output signal at this location can then be regenerated. The signal can be boosted back up to its original energy level. It can be repeated and then continue to propagate on its way to the Receiver/User. This is shown in Figure 1-8.

 

Figure 1-8: Regenerating and repeating an attenuated signal in order to reach the user

Nonetheless, the attenuation characteristics are an item of significant consequence. The Transmission Medium selected in the design must have its attenuation characteristics matched to the Source-User separation. The lower the attenuation in dB/km the greater advantage a Transmission Medium has.
 

1.3 The Transmission Medium - Interference Constraints

Have a Transmitter launch a disturbance into a Transmission Medium. Provide an input data signal to a Transmission Medium. As it propagates down the Transmission Medium it will encounter all sorts of deleterious effects which are termed noise or interference. In the simplest example, that of one person speaking to another person, what we refer to as noise really is what we commonly understand noise to be.

What is noise/interference? It is some extraneous signal that is usually generated outside of the Transmission Medium. Somehow it gets inside of the Transmission Medium. It realizes its effect usually by adding itself to the propagating signal. Though, sometimes it may multiply the propagating signal. The term noise is generally used when this extraneous signal appears to have random amplitude parameters- like background static in AM radio. The term interference is used when this extraneous signal has a more deterministic structure-like 60-cycle hum on a TV set.

In any case, when the Receiver obtains the output signal it must make its decision about what Information it represents in the presence of this noise/interference. It must demodulate the output signal in the presence of noise/interference.

Noise/interference may originate from a variety of sources. Noise/interference may come from the signals generated by equipment located near the transmitter/transmission medium/receiver. This may be equipment that has nothing at all to do with the data link. Such equipment may be motors or air conditioners or automated tools. Noise/interference may come from atmospheric effects. It may arise from using multiple electrical grounds. Noise/interference may be generated by active circuitry in the transmitter and/or receiver. It may come from the operation of other data links.

In obtaining the design solution noise/interference makes its effect best known through the Bit Error Rate (BER). The level of noise/interference drives the BER. Of course, this can be countered by having the Transmitter inject a stronger input signal. It can be countered by having the Receiver be able to detect lower minimal level output signals. But, this comes with greater expense. It does not hide the fact that there is concern with noise/interference because of its impact on the BER.

The susceptibility to noise/interference varies from Transmission Medium to Transmission Medium. Consequently, during the design process attention has to be paid to the Source-User pair. Attention has to be directed to the application underlying the communication needed by this pair and to the BER required by this application.

The Transmission Medium must then be picked that has a noise/interference level capable of delivering the required BER.

1.4 The Transmission Medium- Bandwidth Constraints

Go back and consider the model illustrated in Figure 1-6. Suppose the input signal that the Transmitter sends into the Transmission Medium is the simple cosinusoidal signal of amplitude '1' at frequency 'fo' Hz. The output signal response to this at the Receiver is designated 'T (fo).' Now consider the cosinusoidal test input signal frequency, fo to be varied from 0 Hz on up to ¥. The resulting output signal as a function of frequency is T (fo) or suppressing the subscript- it is T (f). This is referred to as the transfer function of the Transmission Medium. Generally, the ordinate target value 'T (f)' for a given frequency 'f' is referred to as the transfer function gain- actually it is a loss- and is expressed logarithmically in dB relative to the amplitude '1' of the input signal.

One example transfer function is illustrated in Figure 1-9. This is merely an example transfer function. It is not to be understood as to be typical in any sense. It is just an example. However, it does illustrate a feature that is common in the transfer function of any Transmission Medium that can actually be obtained in the real, physical, world. The transfer function rolls off with frequency. The transfer function shown here oscillates, but the maximum value of its oscillation becomes less and less. Yet, the transfer function itself never really rolls off and becomes dead flat zero beyond a certain frequency. This roll off with frequency means that the Transmission Medium attenuates the cosinusoidal signals of the higher frequencies that are given to it as inputs. The energy of these higher frequency signals is somehow lost, usually as heat, in traversing the Transmission Medium. The greater the distance through the Transmission Medium, the more high frequency signals get attenuated. This is a consequence of the greater interaction between the propagating signals and the material comprising the Transmission Medium.

 

Figure 1-9: Example transfer function of a transmission medium

This roll off feature of the transfer function is present in every Transmission Medium regardless of how it is derived. It is present in sound waves. It is present in conductors. It is present in fiber optic cables. It is present in a phonograph record or tape. It is even present in a sheet of writing paper.

The transfer function shown rolls off with frequency. However, most of its activity, most of its area, most of its mass, most of its spread, seems to be below a certain given frequency. In this example it looks like the frequency 'F.' The frequency spread of the transfer function is referred to as its bandwidth. Of course, from what was mentioned above bandwidth decreases with the propagation distance through the Transmission Medium.

Because frequency spread is very subjective the measure of bandwidth is also subjective. When you are discussing communications with someone and they mention bandwidth it isn't such a bad idea to ask exactly how they are defining it. There is a definition in the Glossary in the back of this book. However, it is only one such definition. There are many. For example, there is the 3 dB bandwidth, mean square bandwidth, first lobe bandwidth, brick wall bandwidth and on and on. In a study carried out seventeen years ago the author easily identified over twenty-five separate definitions of bandwidth. All have validity. Whether one is meaningful or not depends upon the context, actually the application, in which it is being used. One definition may be appropriate for describing satellite communication links and another more appropriate for an FCC official considering the request for a broadcast AM radio license.

In any case, a Transmission Medium has a transfer function and the frequency spread of this transfer function is measured by the bandwidth. The bandwidth parameter has implications with respect to the performance of the data link being designed.

In order to see this consider the illustration shown in Figure 1-10. Here the Source is generating data, '0's and '1's every T seconds. Let T= 1/R, in which case the Source is generating data at R bits per second of BPS. To send this data to the User the Transmitter is generating either a positive or negative impulse every T seconds. What is an impulse? It is an infinitesimally narrow pulse, but it is infinitely high so that it has energy of '1.'

Now what comes out at the Receiver in response to the positive impulse sent at time zero to represent the binary data bit '1.' An example result is illustrated in Figure 1-11. Notice that this response out of the Transmission Medium to the input impulse is a pulse spread out in time with its center at t seconds where t is not equal to 0 seconds. This output is only an example. It can not even be called typical. However, it does indicate a property that is typical of all output signals received from the Transmission Medium. The time spreading of the output pulse is this common property. It is called time dispersion. It is a result of the finite bandwidth of the Transmission Medium. To be exact, it is due to the fact that the transfer function of the Transmission Medium- and any Transmission Medium- attenuates the higher signals.

 

Figure 1-10: Binary data from source represented by impulse train put into transmission medium by transmitter. Impulses are T seconds apart.

Look closely at the output signal pulse shown in Figure 1-11. Because it is spread in time it is going to interfere with the output pulses due to input data signals which will come after it. These are not shown in the illustration, but the implication should be clear. Likewise, these subsequent data signals will generate output pulses that will also be spread in time. Each will also interfere with the pulses coming after it and also coming before it. This type of interference is called intersymbol interference. It is not just a consequence of the input signals being impulses. An input signal, of finite duration, and of any shape will generate an output signal with time dispersion.

As the data rate from the Source increases the intersymbol interference problem gets worse and worse. Output pulses with time dispersion get squeezed next to one another. The growing level of intersymbol interference makes it harder and harder for the Receiver to demodulate these signals.

To some extent the intersymbol interference can be undone by sophisticated signal processing in the Receiver. This usually goes under the name of equalization. However, in many cases equalization still can not deliver the data from the Receiver with the BER required by the Source-User pair. In other cases, the data being generated by the Source, say R BPS, is so high that an equalizer can not be obtained fast enough to keep up with the output signals.

 

Figure 1-11: Input signal is positive impulse. Resulting output signal shows time dispersion

In considering the data link design task the first line of defense against time dispersion and intersymbol interference lies in the proper selection of the Transmission Medium. The larger the bandwidth of the Transmission Medium the fewer high frequency components will be attenuated during propagation and the smaller the time dispersion. As a result, there will be less interference between different output pulses. Make no mistake. Intersymbol interference will not disappear. It is just that it will be lessened and made more tolerable as the bandwidth gets larger. In particular, to lessen intersymbol interference the bandwidth of the Transmission Medium must get larger in relation to the Source's generated bit rate, R BPS.

The Transmission Medium must be selected to accommodate the bit rate generated by the Source. This is a critical step in the data link design effort. The Transmission Medium must have sufficient bandwidth so that it will generate tolerable intersymbol interference at the Receiver. This means selecting a Transmission Medium that has a bandwidth that is some multiple of the bit rate, R. A number of rules of thumb are often used to do this. However, they are too specific and not worth discussing at this point especially since the measure of bandwidth is subjective.

The important point is that as the data rate requirement, R, goes up, this limits the selection of Transmission Medium candidates. It limits the selection to those with bandwidths matched to it.

The information technology explosion in the world has made this selection task ever more challenging. Continuously, PCs are becoming more powerful. More complex applications programs can be run and are finding their way into easily usable software. As a result, the Source bit rate requirement is growing at an order of magnitude every few years. To put this in perspective, consider that just ten years ago a Transmission Medium would be quite acceptable if it had a bandwidth matched to a Source bit rate of 9,600 BPS. This Source bit rate was typical of that generated by most data equipment applications. Today with the growing demand for video services and the plethora of graphics in computer applications the demand more often than not is for a Transmission Medium with a bandwidth matched to Source bit rates well upwards of 1 MBPS, possibly 1 GBPS.

1.5 the Transmission Medium - Cost Constraints

You may be able to find the ideal Transmission Medium relative to attenuation, interference and bandwidth. But, you still may not be able to select it as part of the solution to the data link design problem. Why? It simply costs too much. The expense that it presents is beyond the budget allowed for the Source-User communications.

This isn't anything new or revolutionary. Money doesn't drive the world. But, it sure has a tremendous influence on the ultimate choice of solution to any problem based in technology. This was true one hundred years ago and true today.

1.6 Attractiveness of Fiber Optic Cable As A Premises Transmission Medium

Considering this discussion of the constraints on the Transmission Medium we are naturally led to fiber optic cable as an attractive choice for the data link design. Why? When compared with other candidates for the Transmission Medium commonly employed today, there is no comparison when it comes to attenuation, interference and bandwidth.

Illustrations can tell the story best here.

Take a look at Figure 1-12 first. This shows the attenuation of several candidates for the Transmission Medium. All are based on electromagnetic technology and all are in common use today. In other words none are laboratory curiosity items. Attenuation in dB/km is shown as a function of frequency. Here frequency would more or less refer to the data rate from the Source or equivalently the signaling rate from the Transmitter. Attenuation of an electromagnetic Transmission Medium increases with frequency due to effects on an atomic level, which are well beyond this discussion. The attenuation curves of different Transmission Medium candidates are shown as shaded strips because the exact attenuation tends to vary from sample to sample as well as manufacturer to manufacturer. However, the general trend can easily be grasped. The attenuation of the two fiber optic cable types, multi-mode and single mode, are much, much, less than the other candidates. What is more their dependence upon frequency is even flat over quite a large range. This makes designing data links with them simpler. You need not be concerned with the change in attenuation every time you decide to tweak the data rate.

To be absolutely clear the fiber optic cable attenuation shown in this figure is for fiber optic cable fabricated totally from glass (silica). That is, it has a glass core and glass cladding. There is also fiber optic cable fabricated totally from plastic and fiber optic cable having a glass-silica core with a plastic cladding (PCS- Plastic Clad Silica). It is the pure glass- silica based fiber optic cable that has the low attenuation properties. The plastic based fiber optic cable has much higher attenuation, well above coaxial cable. But, it does have some attractive features that will be discussed in a later chapter.

 

Figure 1-12: Attenuation versus frequency (Courtesy of Siecor Corporation)

You get the idea. When it comes to considering the attenuation issue then fiber optic cable is the unchallenged selection for the Transmission Medium.

Fiber optic cable is fabricated from glass or plastic. Because of the nature of this material it allows signals transmitted through fiber optic cable to be immune from electromagnetic based forms of noise and interference. This includes power transients that may arise from lightning strikes. It includes noise arising from ground loops. In fact, fiber optic cable provides nearly perfect isolation between multiple grounds. Noise can still affect a fiber optic data link; especially, if it is generated in the receiver or transmitter electronic circuitry. However, the effect of noise and interference originating outside the link is far less than with competing choices for the Transmission Medium, candidates like shielded or unshielded twisted pair cable or coaxial cable or free space microwave radio.

Take a look at Figure 1-13. This illustrates the variation of the bandwidth of fiber optic cable with its length. Remember bandwidth goes down with increasing length. But, that is not the concern here. Notice that at up to 4 km the bandwidth is always above 10 MHz. This implies that a fiber optic link can support data rates of many 10's of MBPS over these distances. This can be done without having to have the Transmitter resort to any sophisticated bandwidth efficient modulation schemes. Of course, people talk about fiber optic cable being able to support Giga Bits Per Second (1 Billion Bits Per Second - GBPS) and even Tera Bits Per Second (1 Trillion Bits Per Second). But, remember this depends upon distance and may often require multiple repeaters.

 

Figure 1-13: Bandwidth of fiber optic cable vs. length (from Fiber Optic Communications, Joseph C. Palais)

To put this in perspective, unshielded twisted pair copper cable over this distance can support 0-to-100 MBPS. Coaxial cable this distance can support about 20 MBPS. When it comes to the bandwidth issue fiber optic cable is the unquestioned most attractive candidate for the Transmission Medium.

Fiber optic cable is the unchallenged winner in the Transmission Medium sweepstakes when it comes to attenuation, interference and bandwidth. It even has some additional features that are attractive in comparing it to other candidates mentioned. It is the most secure. Tampering with fiber optic with transmissions through fiber optic cable is very difficult to do. It can be detected far more easily than with the other metallic based candidates for Transmission Medium let alone free space propagation candidates. The small size of fiber optic cables allows it to be placed in ducting that is too small for metallic cable. This allows room for substantial growth in capacity if needed. It's easier to put more fiber optic cables in the same duct. This is brought out in the photograph provided in Figure 1-14. Finally, fiber optic cables do not conduct electricity- they are glass or plastic therefore safer. They are particularly suitable for use in areas that might have spark or electrical hazard restrictions. This is especially true of places that may endanger the well being of a technician working with a long segment of metallic cable instead of a fiber.

 

Figure 1-14: Size comparison: coaxial cable and fiber optic cable (Courtesy of AT&T Archives)

Undoubtedly now you are saying So fiber optic cable is the winner when it comes to attenuation, interference and bandwidth. But, doesn't high cost throw it out? Isn't it very expensive and wasn't this the ultimate driver for the Transmission Medium selection?

It is true when comparing fiber optic cable to other candidates it is not as attractive from a cost point of view. However, the situation is getting better year by year. In particular take a look at Figure1-15. This illustrates the cost trends for different candidates for the Transmission Medium. Cost trends are graphed for the period 1990 through 1995. Notice the decrease for fiber optic cable. In the years since it has decreased even further. Of course, this is for glass based fiber optic cable. Plastic fiber optic cable has a much lower cost. In any case from a cost point of view fiber optic cable is and will probably continue to be more expensive than the cheapest, voice grade, unshielded twisted pair cable. However, its cost is merging with the other candidates. Certainly, the really minor cost disadvantage is greatly outweighed with the significant performance advantages.

 

Figure 1-15: Cost trends of common transmission media

Putting this altogether there is no argument. Fiber optic cable should be the Transmission Medium of choice when considering data links in new facilities where no other Transmission Medium candidate exists.

There is and will continue to be tremendous activity with respect to carrying out data communications in the wide area network or long haul environment. This is the environment of the long distance carrier, the Telephone Company.

However, there is even greater activity with respect to the implementation of data links in the premises or local area environment. This is the environment of the office building, Small Office Home Office (SOHO), the factory and the campus. As PC's have proliferated throughout all premise type facilities the need for data communications links has followed. Installation of premises data links be they point-to-point, multi-point, part of a Local Area Network (LAN) or whatever is a major agenda item for many business concerns. The case has been made above for fiber optic cable being the Transmission Medium of choice for these links. This is why it is the subject of interest in this book.

1.7 Program

This book has been written so that each chapter stands on its own. There is no need to read the chapters in order. There may be occasionally cross-references from one chapter to another. However, the information can easily be gleaned without going back to the very beginning.

A brief summary of the sequel is as follows:

Chapter 2 - A careful review is given to the details of a fiber optic data link for the premise environment. The possibilities for and properties of fiber optic cable are discussed. Candidates for the Transmitter and Receiver are considered. Connectors and splices are introduced. The performance of the data link is analyzed with a careful look at the loss budget.

Chapter 3 - Consideration is given to exploiting the large bandwidth presented by fiber optic cable to support the data communications of multiple users - multiple Source - User pairs. That is, how to carve out multiple fiber optic data links from a single fiber optic cable in the premises environment. This is accomplished by multiplexing. Both Time Division Multiplexing (TDM) and Wavelength Division Multiplexing (WDM) are discussed.

Chapter 4 - Discussion focuses on the Local Area Network (LAN). Fiber optic data links are joined with LAN's. Using LAN architectures carries out a great deal of premise data communication. The delay properties of fiber optic cable can be exploited to extend the distance coverage of a LAN. A fiber optic data link can be used to connect remote stations to a LAN hub. Stations that may be too far from a LAN to be connected by a copper cable may possibly be joined by a fiber optic data link.

Chapter 5 - The manufacturing environment is considered. In particular the environment presented by heavy industry that always has a plethora of high (electric) powered tools in use. The manufacturing environment presents a situation where premises data communications may have to be carried out with intense noise and interference present. The interference protection properties of a fiber optic data link are considered in this environment. In particular, consideration is given to the types of data links and networking architectures generally found in the manufacturing environment. The discussion centers on how these links and architectures can exploit the interference protection properties of a fiber optic data link.

Chapter 6 - Discussion centers on fiber optic products that can be used to serve serial data communications.

Chapter 7 - Standards that cover the use of fiber optic data links within premises networks are enumerated. Organization from which they can be ordered, in full, are provided.

Chapter 8 - A glossary that covers the subject of fiber optic data communications. It provides terminology specifically covered within this book. However, it goes further and provides terminology that may not be used here but may be encountered within a broader view of the interest area or within communications in general.

CHAPTER 2

THE FIBER OPTIC DATA COMMUNICATIONS LINK FOR THE PREMISES ENVIRONMENT

2.1 The Fiber Optic Data Communications Link, End-to-End

In this chapter we consider the simple fiber optic data link for the premises environment. This is the basic building block for a fiber optic based network. A model of this simple link is shown in Figure 2-1.

 

Figure 2-1: Model of "simple" fiber optic data link

The illustration indicates the Source-User pair, Transmitter and Receiver. It also clearly shows the fiber optic cable constituting the Transmission Medium as well as the connectors that provide the interface of the Transmitter to the Transmission Medium and the Transmission Medium to the Receiver.

All of these are components of the simple fiber optic data link. Each will be discussed. Consideration will be in the following order: fiber optic cable, Transmitter, Receiver and connectors. We will conclude by taking up the question of how to analyze the performance of the simple fiber optic data link.

2.2 Fiber Optic Cable

We begin by asking Just what is a fiber optic cable? A fiber optic cable is a cylindrical pipe. It may be made out of glass or plastic or a combination of glass and plastic. It is fabricated in such a way that this pipe can guide light from one end of it to the other.

The idea of having light guided through bent glass is not new or high tech. The author was once informed that Leonardo DaVinci actually mentioned such a means for guiding light in one of his notebooks. However, he has not been able to verify this assertion. What is known for certain is that total internal reflection of light in a beam of water - essentially guided light - was demonstrated by the physicist John Tyndall [1820-1893] in either 1854 or 1870 - depending upon which reference you consult. Tyndall showed that light could be bent around a corner while it traveled through a jet of pouring water.

Using light for communications came after this. Alexander Graham Bell [1847-1922] invented the photo-phone around 1880. Bell demonstrated that a membrane in response to sound could modulate an optical signal, light. But, this was a free space transmission system. The light was not guided.

Guided optical communications had to wait for the 20th century. The first patent on guided optical communications over glass was obtained by AT &T in 1934. However, at that time there were really no materials to fabricate a glass (or other type of transparent material) fiber optic cable with sufficiently low attenuation to make guided optical communications practical. This had to wait for about thirty years.

During the 1960's researchers working at a number of different academic, industrial and government laboratories obtained a much better understanding of the loss mechanisms in glass fiber optic cable. Between 1968 and 1970 the attenuation of glass fiber optic cable dropped from over 1000 dB/km to less than 20 dB/km. Corning patented its fabrication process for the cable. The continued decrease in attenuation through the 1970's allowed practical guided light communications using glass fiber optic cable to take off. In the late 1980's and 1990's this momentum increased with the even lower cost plastic fiber optic cable and Plastic Clad Silica (PCS).

Basically, a fiber optic cable is composed of two concentric layers termed the core and the cladding. These are shown on the right side of Figure 2-2. The core and cladding have different indices of refraction with the core having n₁ and the cladding n₂. Light is piped through the core. A fiber optic cable has an additional coating around the cladding called the jacket. Core, cladding and jacket are all shown in the three dimensional view on the left side of Figure 2-2. The jacket usually consists of one or more layers of polymer. Its role is to protect the core and cladding from shocks that might affect their optical or physical properties. It acts as a shock absorber. The jacket also provides protection from abrasions, solvents and other contaminants. The jacket does not have any optical properties that might affect the propagation of light within the fiber optic cable.

The illustration on the left side of Figure 2-2 is somewhat simplistic. In actuality, there may be a strength member added to the fiber optic cable so that it can be pulled during installation.

 

Figure 2-2: Fiber Optic Cable, 3 dimensional view and basic cross section

This would be added just inside the jacket. There may be a buffer between the strength member and the cladding. This protects the core and cladding from damage and allows the fiber optic cable to be bundled with other fiber optic cables. Neither of these is shown.

How is light guided down the fiber optic cable in the core? This occurs because the core and cladding have different indices of refraction with the index of the core, n₁, always being greater than the index of the cladding, n₂. Figure 2-3 shows how this is employed to effect the propagation of light down the fiber optic cable and confine it to the core.

As illustrated a light ray is injected into the fiber optic cable on the right. If the light ray is injected and strikes the core-to-cladding interface at an angle greater than an entity called the critical angle then it is reflected back into the core. Since the angle of incidence is always equal to the angle of reflection the reflected light will again be reflected. The light ray will then continue this bouncing path down the length of the fiber optic cable. If the light ray strikes the core-to-cladding interface at an angle less than the critical angle then it passes into the cladding where it is attenuated very rapidly with propagation distance.

Light can be guided down the fiber optic cable if it enters at less than the critical angle. This angle is fixed by the indices of refraction of the core and cladding and is given by the formula:

Q_c = arc cosine (n₂ /n₁).

The critical angle is measured from the cylindrical axis of the core. By way of example, if n₁ = 1.446 and n₂= 1.430 then a quick computation will show that the critical angle is 8.53 degrees, a fairly small angle.

Of course, it must be noted that a light ray enters the core from the air outside, to the left of Figure 2-3. The refractive index of the air must be taken into account in order to assure that a light ray in the core will be at an angle less than the critical angle. This can be done fairly simply. The following basic rule then applies. Suppose a light ray enters the core from the air at an angle less than an entity called the external acceptance angle - Q_ext It will be guided down the core. Here

Q_ext = arc sin [(n₁/ n₀) sin (Q_c)]

with n0 being the index of refraction of air. This angle is, likewise, measured from the cylindrical axis of the core. In the example above a computation shows it to be 12.4 degrees - again a fairly small angle.

 

Figure 2-3: Propagation of a light ray down a fiber optic cable

Fiber optic data link performance is a subject that will be discussed in full at the end of this chapter. However, let's jump the gun just a little. In considering the performance of a fiber optic data link the network architect is interested in the effect that the fiber optic cable has on overall link performance. Consideration of performance comes to answering three questions:

1) How much light can be coupled into the core through the external acceptance angle?

2) How much attenuation will a light ray experience in propagating down the core?

3) How much time dispersion will light rays representing the same input pulse experience in propagating down the core?

The more light that can be coupled into the core the more light will reach the Receiver and the lower the BER. The lower the attenuation in propagating down the core the more light reaches the Receiver and the lower the BER. The less time dispersion realized in propagating down the core the faster the signaling rate and the higher the end-to-end data rate from Source-to-User.

The answers to these questions depend upon many factors. The major factors are the size of the fiber, the composition of the fiber and the mode of propagation.

When it comes to size, fiber optic cables have exceedingly small diameters. Figure 2-4 illustrates the cross sections of the core and cladding diameters of four commonly used fiber optic cables. The diameter sizes shown are in microns, 10-6 m. To get some feeling for how small these sizes actually are, understand that a human hair has a diameter of 100 microns. Fiber optic cable sizes are usually expressed by first giving the core size followed by the cladding size. Consequently, 50/125 indicates a core diameter of 50 microns and a cladding diameter of 125 microns; 100/140 indicates a core diameter of 100 microns and a cladding diameter of 140 microns. The larger the core the more light can be coupled into it from external acceptance angle cone. However, larger diameter cores may actually allow too much light in and too much light may cause Receiver saturation problems. The left most cable shown in Figure 2-4, the 125/8 cable, is often found when a fiber optic data link operates with single-mode propagation. The cable that is second from the right in Figure 2-4, the 62.5/125 cable, is often found in a fiber optic data link that operates with multi-mode propagation.

 

Figure 2-4: Typical core and cladding diameters -Sizes are in microns

When it comes to composition or material makeup fiber optic cables are of three types: glass, plastic and Plastic Clad Silica (PCS). These three candidate types differ with respect to attenuation and cost. We will describe these in detail. Attenuation and cost will first be mentioned only qualitatively. Later, toward the end of this sub-chapter the candidates will be compared quantitatively.

By the way, attenuation is principally caused by two physical effects, absorption and scattering. Absorption removes signal energy in the interaction between the propagating light (photons) and molecules in the core. Scattering redirects light out of the core to the cladding. When attenuation for a fiber optic cable is dealt with quantitatively it is referenced for operation at a particular optical wavelength, a window, where it is minimized.

Glass fiber optic cable has the lowest attenuation and comes at the highest cost. A pure glass fiber optic cable has a glass core and a glass cladding. This candidate has, by far, the most wide spread use. It has been the most popular with link installers and it is the candidate with which installers have the most experience. The glass employed in a fiber optic cable is ultra pure, ultra transparent, silicon dioxide or fused quartz. One reference put this in perspective by noting that "if seawater were as clear as this type of fiber optic cable then you would be able to see to the bottom of the deepest trench in the Pacific Ocean." During the glass fiber optic cable fabrication process impurities are purposely added to the pure glass so as to obtain the desired indices of refraction needed to guide light. Germanium or phosphorous are added to increase the index of refraction. Boron or fluorine is added to decrease the index of refraction. Other impurities may somehow remain in the glass cable after fabrication. These residual impurities may increase the attenuation by either scattering or absorbing light.

Plastic fiber optic cable has the highest attenuation, but comes at the lowest cost. Plastic fiber optic cable has a plastic core and plastic cladding. This fiber optic cable is quite thick. Typical dimensions are 480/500, 735/750 and 980/1000. The core generally consists of PMMA (polymethylmethacrylate) coated with a fluropolymer. Plastic fiber optic cable was pioneered in Japan principally for use in the automotive industry. It is just beginning to gain attention in the premises data communications market in the United States. The increased interest is due to two reasons. First, the higher attenuation relative to glass may not be a serious obstacle with the short cable runs often required in premise networks. Secondly, the cost advantage sparks interest when network architects are faced with budget decisions. Plastic fiber optic cable does have a problem with flammability. Because of this, it may not be appropriate for certain environments and care has to be given when it is run through a plenum. Otherwise, plastic fiber is considered extremely rugged with a tight bend radius and the ability to withstand abuse.

Plastic Clad Silica (PCS) fiber optic cable has an attenuation that lies between glass and plastic and a cost that lies between their cost as well. Plastic Clad Silica (PCS) fiber optic cable has a glass core which is often vitreous silica while the cladding is plastic - usually a silicone elastomer with a lower refractive index. In 1984 the IEC standardized PCS fiber optic cable to have the following dimensions: core 200 microns, silicone elastomer cladding 380 microns, jacket 600 microns. PCS fabricated with a silicone elastomer cladding suffers from three major defects. It has considerable plasticity. This makes connector application difficult. Adhesive bonding is not possible and it is practically insoluble in organic solvents. All of this makes this type of fiber optic cable not particularly popular with link installers. However, there have been some improvements in it in recent years.

When it comes to mode of propagation fiber optic cable can be one of two types, multi-mode or single-mode. These provide different performance with respect to both attenuation and time dispersion. The single-mode fiber optic cable provides the better performance at, of course, a higher cost.

In order to understand the difference in these types an explanation must be given of what is meant by mode of propagation.

Light has a dual nature and can be viewed as either a wave phenomenon or a particle phenomenon (photons). For the present purposes consider it as a wave. When this wave is guided down a fiber optic cable it exhibits certain modes. These are variations in the intensity of the light, both over the cable cross section and down the cable length. These modes are actually numbered from lowest to highest. In a very simple sense each of these modes can be thought of as a ray of light. Although, it should be noted that the term ray of light is a hold over from classical physics and does not really describe the true nature of light.

In any case, view the modes as rays of light. For a given fiber optic cable the number of modes that exist depend upon the dimensions of the cable and the variation of the indices of refraction of both core and cladding across the cross section. There are three principal possibilities. These are illustrated in Figure 2-5.

Consider the top illustration in Figure 2-5. This diagram corresponds to multi-mode propagation with a refractive index profile that is called step index. As can be seen the diameter of the core is fairly large relative to the cladding. There is also a sharp discontinuity in the index of refraction as you go from core to cladding. As a result, when light enters the fiber optic cable on the right it propagates down toward the left in multiple rays or multiple modes. This yields the designation multi-mode. As indicated the lowest order mode travels straight down the center. It travels along the cylindrical axis of the core. The higher modes represented by rays, bounce back and forth, going down the cable to the left. The higher the mode the more bounces per unit distance down to the left.

Over to the left of this top illustration are shown a candidate input pulse and the resulting output pulse. Note that the output pulse is significantly attenuated relative to the input pulse. It also suffers significant time dispersion. The reasons for this are as follows. The higher order modes, the bouncing rays, tend to leak into the cladding as they propagate down the fiber optic cable. They lose some of their energy into heat. This results in an attenuated output signal. The input pulse is split among the different rays that travel down the fiber optic cable. The bouncing rays and the lowest order mode, traveling down the center axis, are all traversing paths of different lengths from input to output. Consequently, they do not all reach the right end of the fiber optic cable at the same time. When the output pulse is constructed from these separate ray components the result is time dispersion.

 

Figure 2-5: Types of mode propagation in fiber optic cable (Courtesy of AMP Incorporated)

Fiber optic cable that exhibits multi-mode propagation with a step index profile is thereby characterized as having higher attenuation and more time dispersion than the other propagation candidates have. However, it is also the least costly and in the premises environment the most widely used. It is especially attractive for link lengths up to 5 km. Usually, it has a core diameter that ranges from 100 microns to 970 microns. It can be fabricated either from glass, plastic or PCS.

Consider the middle illustration in Figure 2-5. This diagram corresponds to single-mode propagation with a refractive index profile that is called step index. As can be seen the diameter of the core is fairly small relative to the cladding. Typically, the cladding is ten times thicker than the core. Because of this when light enters the fiber optic cable on the right it propagates down toward the left in just a single ray, a single-mode, and the lowest order mode. In extremely simple terms this lowest order mode is confined to a thin cylinder around the axis of the core. (In actuality it is a little more complex). The higher order modes are absent. Consequently, there is no energy lost to heat by having these modes leak into the cladding. They simply are not present. All energy is confined to this single, lowest order, mode. Since the higher order mode energy is not lost, attenuation is not significant. Also, since the input signal is confined to a single ray path, that of the lowest order mode, there is little time dispersion, only that due to propagation through the non-zero diameter, single mode cylinder.

Single mode propagation exists only above a certain specific wavelength called the cutoff wavelength.

To the left of this middle illustration is shown a candidate input pulse and the resulting output pulse. Comparing the output pulse and the input pulse note that there is little attenuation and time dispersion.

Fiber optic cable that exhibits single-mode propagation is thereby characterized as having lower attenuation and less time dispersion than the other propagation candidates have. Less time dispersion of course means higher bandwidth and this is in the 50 to 100 GHz/ km range. However, single mode fiber optic cable is also the most costly in the premises environment. For this reason, it has been used more with Wide Area Networks than with premises data communications. It is attractive more for link lengths go all the way up to 100 km. Nonetheless, single-mode fiber optic cable has been getting increased attention as Local Area Networks have been extended to greater distances over corporate campuses. The core diameter for this type of fiber optic cable is exceedingly small ranging from 5 microns to 10 microns. The standard cladding diameter is 125 microns.

Single-mode fiber optic cable is fabricated from glass. Because of the thickness of the core, plastic cannot be used to fabricate single-mode fiber optic cable. The author is unaware of PCS being used to fabricate it.

It should be noted that not all single-mode fibers use a step index profile. Some use more complex profiles to optimize performance at a particular wavelength.

Consider the bottom illustration in Figure 2-5. This corresponds to multi-mode propagation with a refractive index profile that is called graded index. Here the variation of the index of refraction is gradual as it extends out from the axis of the core through the core to the cladding. There is no sharp discontinuity in the indices of refraction between core and cladding. The core here is much larger than in the single-mode step index case discussed above. Multi-mode propagation exists with a graded index. However, as illustrated the paths of the higher order modes are somewhat confined. They appear to follow a series of ellipses. Because the higher mode paths are confined the attenuation through them due to leakage is more limited than with a step index. The time dispersion is more limited than with a step index, therefore, attenuation and time dispersion are present, just limited.

To the left of this bottom illustration is shown a candidate input pulse and the resulting output pulse. When comparing the output pulse and the input pulse, note that there is some attenuation and time dispersion, but not nearly as great as with multi-mode step index fiber optic cable.

Fiber optic cable that exhibits multi-mode propagation with a graded index profile is thereby characterized as having attenuation and time dispersion properties somewhere between the other two candidates. Likewise its cost is somewhere between the other two candidates. Popular graded index fiber optic cables have core diameters of 50, 62.5 and 85 microns. They have a cladding diameter of 125 microns - the same as single-mode fiber optic cables. This type of fiber optic cable is extremely popular in premise data communications applications. In particular, the 62.5/125 fiber optic cable is the most popular and most widely used in these applications.

Glass is generally used to fabricate multi-mode graded index fiber optic cable. However, there has been some work at fabricating it with plastic.

The illustration Figure 2-6 provides a three dimensional view of multi-mode and single-mode propagation down a fiber optic cable. Table 2-1 provides the attenuation and bandwidth characteristics of the different fiber optic cable candidates. This table is far from being all inclusive, however, the common types are represented.

 

Figure 2-6: Three dimensional view, optical power in multi-mode and single-mode fibers

 

* Too high to measure accurately. Effectively infinite.

Table 2-1: Attenuation and Bandwidth characteristics of different fiber optic cable candidates

Figure 2-7 illustrates the variation of attenuation with wavelength taken over an ensemble of fiber optic cable material types. The three principal windows of operation, propagation through a cable, are indicated. These correspond to wavelength regions where attenuation is low and matched to the ability of a Transmitter to generate light efficiently and a Receiver to carry out detection. The 'OH' symbols indicate that at these particular wavelengths the presence of Hydroxyl radicals in the cable material cause a bump up in attenuation. These radicals result from the presence of water. They enter the fiber optic cable material through either a chemical reaction in the manufacturing process or as humidity in the environment. The illustration Figure 2-8 shows the variation of attenuation with wavelength for, standard, single-mode fiber optic cable.

 

Figure 2-7: Attenuation vs. Wavelength

 

 

Figure 2-8: Attenuation spectrum of standard single-mode fiber

2.3 Transmitter

The Transmitter component of Figure 2-1 serves two functions. First, it must be a source of the light coupled into the fiber optic cable. Secondly, it must modulate this light so as to represent the binary data that it is receiving from the Source. With the first of these functions it is merely a light emitter or a source of light. With the second of these functions it is a valve, generally operating by varying the intensity of the light that it is emitting and coupling into the fiber.

Within the context of interest in this book the Source provides the data to the Transmitter as some digital electrical signal. The Transmitter can then be thought of as Electro-Optical (EO) transducer.

First some history. At the dawn of fiber optic data communications twenty-five years ago, there was no such thing as a commercially available Transmitter. The network architect putting together a fiber optic data link had to design the Transmitter himself. Everything was customized.

The Transmitter was typically designed using discrete electrical and Electro-optical devices. This very quickly gave way to designs based upon hybrid modules containing integrated circuits, discrete components (resistors and capacitors) and optical source diodes (light emitting diodes-LED's or laser diodes). The modulation function was generally performed using separate integrated circuits and everything was placed on the same printed circuit board.

By the 1980's higher and higher data transmission speeds were becoming of interest to the data link architect. The design of the Transmitter while still generally customized became more complex to accommodate these higher speeds. A greater part of the Transmitter was implemented using VLSI circuits and attention was given to minimizing the number of board interconnects. Intense research efforts were undertaken to integrate the optical source diode and the transistor level circuits needed for modulation on a common integrated circuit substrate, without compromising performance. At present, the Transmitter continues to be primarily designed as a hybrid unit, containing both discrete components and integrated circuits in a single package.

By the late 1980's commercially available Transmitter's became available. As a result, the link design could be kept separate from the Transmitter design. The link architect was relieved from the need to do high-speed circuit design or to design proper bias circuits for optical diodes. The Transmitter could generally be looked at as a black box selected to satisfy certain requirements relative to power, wavelength, data rate, bandwidth, etc. This is where the situation remains today.

To do a proper selection of a commercially available Transmitter you have to be able to know what you need in order to match your other link requirements. You have to be able to understand the differences between Transmitter candidates. There are many. We can not begin to approach this in total.

However, we can look at this in a limited way. Transmitter candidates can be compared on the basis of two characteristics. Transmitter candidates can be compared on the basis of the optical source component employed and the method of modulation.

Let us deal with the optical source component of the Transmitter first. This has to meet a number of requirements. These are delineated below:

First, its physical dimensions must be compatible with the size of the fiber optic cable being used. This means it must emit light in a cone with cross sectional diameter 8-100 microns, or it can not be coupled into the fiber optic cable.

Secondly, the optical source must be able to generate enough optical power so that the desired BER can be met.

Thirdly, there should be high efficiency in coupling the light generated by the optical source into the fiber optic cable.

Fourthly, the optical source should have sufficient linearity to prevent the generation of harmonics and intermodulation distortion. If such interference is generated it is extremely difficult to remove. This would cancel the interference resistance benefits of the fiber optic cable.

Fifthly, the optical source must be easily modulated with an electrical signal and must be capable of high-speed modulation-or else the bandwidth benefits of the fiber optic cable are lost.

Finally, there are the usual requirements of small size, low weight, low cost and high reliability. The light emitting junction diode stands out as matching these requirements. It can be modulated at the needed speeds. The proper selection of semiconductor materials and processing techniques results in high optical power and efficient coupling of it to the fiber optic cable. These optical sources are easily manufactured using standard integrated circuit processing. This leads to low cost and high reliability.

There are two types of light emitting junction diodes that can be used as the optical source of the Transmitter. These are the light emitting diode (LED) and the laser diode (LD). This is not the place to discuss the physics of their operation. LED's are simpler and generate incoherent, lower power, light. LD's are more complex and generate coherent, higher power light. Figure 2-9 illustrates the optical power output, P, from each of these devices as a function of the electrical current input, I, from the modulation circuitry. As the figure indicates the LED has a relatively linear P-I characteristic while the LD has a strong non-linearity or threshold effect. The LD may also be prone to kinks where the power actually decreases with increasing bandwidth.

With minor exceptions, LDs have advantages over LED's in the following ways.

• They can be modulated at very high speeds.
• They produce greater optical power.
• They have higher coupling efficiency to the fiber optic cable.

LED's have advantages over LD's because they have

• higher reliability
• better linearity